Field of the Invention
The present invention relates to an article having antifouling properties and intended to be employed in aquatic uses, in particular marine uses, and also to a method for delaying the growth of aquatic organisms on submersible or semi-submersible structures.
The invention relates to the field of antifouling marine paints. Antifouling marine paints are topcoats intended to prevent the attachment of animals or plants to the lower parts of the hull of ships. They are used for reasons of safety, maintaining the maneuverability of ships, reducing fuel consumption, combating corrosion and weighing-down of structures.
Description of Related Art
The problem of “biofouling” constitutes a major problem resulting from the immersion of materials in marine environments. The prevention of this phenomenon represents a considerable maintenance cost.
Specifically, the formation of “biofouling” or “fouling” occurs during immersion in seawater, where a layer of organic and inorganic molecules is adsorbed to the surface of the material extremely rapidly. This layer of adsorbed material, or biofilm, serves as a mediator for the adhesion of the bacteria present in suspension in the marine environment.
This colonization of the surface by marine bacteria is rapid and a stationary state is reached after a period of a few hours to a few days. Finally, other marine organisms colonize the surface, the adherent bacteria recruiting these other organisms. All these live organisms attached to the surface constitute the biofouling or fouling.
The adhesion of marine fouling concerns any structure immersed in the sea: ships, pipelines, cooling towers and circuits, harbor structures, marine sensors, aquaculture systems, etc. The damage caused is considerable and diverse. Specifically, the structures become coated, for example, with organisms which have a negative effect on the performance levels of the structures.
In particular, for the hulls of ships, the incrustation of various marine organisms increases the friction between the ships' hulls and the seawater, which reduces the speed and can lead to greater fuel consumption. Thus, the bottom of a ship which is not protected by an antifouling system can, after less than six months spent at sea, be covered with 150 kg of fouling per square meter.
In order to avoid this economic loss, and also in order to more successfully inhibit corrosion phenomena, antifouling paints, the objective of which is to prevent or notably reduce the soiling due to the incrustations of marine organisms, are applied to the immersed parts of the structures exposed to water. The principle of antifouling paints is based on the controlled release of the active substance at the interface between the surface and the seawater. The effectiveness of the paint is maintained as long as the concentration of active substance released at the surface is effective and regular. Most antifouling paints therefore contain a biocidal product which is most commonly an organometallic compound (based on tin, on copper or on zinc) or an organic compound (fungicide, algicide, bactericide) which prevents adhesion of the marine soiling owing to the toxic activity thereof.
However, the problem associated with the use of these paints is that they release into the marine environment substances that are harmful to the maritime fauna and flora. In addition, the coatings become increasingly rough and gradually degrade, which increases fuel consumption and increases the hydrodynamic noise emitted by the immersed structure.
This new difficulty has been solved by using self-polishing antifouling paints. In addition to having biocidal agents, these paints exhibit, under the action of surface hydrolysis by the seawater and that of erosion due to the movement of the ship, a regular and controlled loss of thickness over time. The slow erosion of the coating in contact with the seawater makes it possible to constantly refresh the surface with biodical agents.
The self-polishing antifouling paints developed since the 1960s were based on tin salts. They were self-polishing paints formulated from tributyltin (TBT) methacrylate copolymers which have a constant degree of leaching. The TBT grafted to an acrylic binder is released slowly by hydrolysis in water. Examples of this type of paint are described in documents FR-A-2266733, FR-A-2557585, EP-A-0051930 and GB-A-2118196.
Tributyltin (TBT), which is very effective, was therefore the biocide most commonly used in antifouling paints, but this product, its degradation molecules and its metabolites proved to be seriously and sustainably polluting. For these reasons, the International Maritime Organization prohibited the use of tin-based antifouling paints.
The antifouling paints used today are mainly based on copper-containing compounds and/or on synthetic chemical compounds, but also based on silicone polymers.
With regards to the copper-based paints, although they are less toxic than tin salts, they are virtually always formulated with a massive proportion of cuprous oxide (see, for example, document EP-A-051930 or FR-A-2557585), the main binder being based on special polymers generally of the acrylic type. However, they are effective only against the marine fauna, and, in order to combat the growth of algae, it is essential to add herbicides, which can pose new threats to the environment.
This alternative does not therefore provide a sustainable solution for protecting the environment against the considerable discarding of heavy ions, in particular copper ions, following the intensive use of paints which are tin-free but are rich in copper.
Another solution for preventing the soiling of the surfaces of structures in contact with seawater consists in covering these surfaces with at least one protective coating, the external layer of the coating in contact with the water being a silicone elastomer. These coatings are prepared using paints known as “fouling-release coating”. The principle of these new antifouling paints is to create a very smooth surface, with a low surface energy, to which the organisms have great difficulty in adhering. When such surfaces are stationary, marine organisms can deposit themselves thereon. However, by virtue of the flexibility and of the low surface tension of the silicone-based topcoat, these organisms are quite simply removed by the force of the movement of the water or the effect of friction caused by the movement of the ship. This also means that, if there is sufficient movement of water around the hull of a ship, a natural self-cleaning effect occurs.
By virtue of these properties, even ships which are less frequently at sea or in waters with less movement benefit from cleaning intervals which are more spaced out. This is due to the fact that the marine organisms have trouble adhering to the surface; which also makes the cleaning easier.
These silicone-based paints forming an antifouling coating are therefore very innovative:                they are completely friendly to the marine environment: no metal waste, and        they improve the glide of ships, thus reducing by 1 to 5% their fuel consumption and therefore their greenhouse gas emissions.        
There are many patents, for example patents FR-A-2 083 029 and U.S. Pat. No. 3,702,778, describing such coatings of which the topcoat is a hot-cured or cold-cured silicone elastomer.
For example, U.S. patent application Ser. No. 07/847,401, filed on Mar. 6, 1992, describes a three-component antifouling system comprising at least one epoxy primer coat, one adhesion primer coat (tie coat) and one antifouling coat (topcoat) based on a silicone elastomer. The final epoxy primer coat is normally a thin coat which is applied in order to obtain a clean and fresh surface to which the tie coat can adhere. The tie coat comprises an organopolysiloxane and a curing component. The antifouling coat comprises an organpolysiloxane, an alkyl silicate, a curing agent and a tin-based catalyst. The epoxy primer coat(s) is (are) applied directly to the support. The tie coat is applied to the epoxy primer coat(s). The antifouling coat, as a silicone coating, is then applied and crosslinked on the tie coat, after partial curing of the latter.
An antifouling coat (topcoat) based on a silicone elastomer can also comprise exuding compounds which improve the “antifouling” effect, in particular:                methylphenylpolysiloxane oils (U.S. Pat. No. 4,025,693),        a hydrocarbon-based liquid compound, for example a polyolefin,        a plasticizer,        a lubricating oil (FR-A-2 375 305),        liquid paraffins and waxy masses of the petrolatum type (JP-A-83/013 673),        a thermoplastic polymer such as PVC,        a vinyl chloride/vinyl acetate copolymer (Kokai JP-A-79/026 826), or        cationic, anionic, nonionic or amphoteric surfactants (JP-A-85/258 271).        
In order to form the silicone elastomer coating, the silicone formulations used generally involve a silicone oil, generally a reactive polydimethylsiloxane with hydroxylated endings, which optionally prefunctionalize with a silane so as to have alkoxy ends, a crosslinking agent and a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and optional other additives, such as bulking fillers, adhesion promoters, dyes, etc.
These room-temperature vulcanizing organopolysiloxane compositions are well known and are classified in 2 distinct groups: single-component compositions (RTV-1) and two-component compositions (RTV-2). The term “RTV” is the acronym for “room-temperature vulcanizing”.
During crosslinking, water (either provided by atmospheric moisture in the case of RTV-1 compositions, or introduced into one part of the composition in the case of RTV-2 compositions) enables the polycondensation reaction, which results in the formation of the elastomeric network.
Generally, single-component (RTV-1) compositions crosslink when they are exposed to moisture from the air, i.e. they cannot crosslink in an enclosed medium. For example, the single-component silicone compositions cold-crosslink according to a mechanism of hydrolysis of reactive functions of the acetoxysilane, ketiminoxysilane, alkoxysilane, etc., type, followed by condensation reactions between silanol groups formed and other residual reactive functions. The hydrolysis is generally carried out by virtue of the water vapor which diffuses into the material from the surface exposed to the atmosphere. Generally, the kinetics of the polycondensation reactions are extremely slow; these reactions are therefore catalyzed by a suitable catalyst. As catalysts which are used, use is most commonly made of catalysts based on tin, on titanium, on an amine or compositions of these catalysts. Catalysts based on tin (cf. in particular FR-A-2 557 582) and on titanium (cf. in particular FR-A-2 786 497) are catalysts that are very effective. Single-component silicone elastomers with —Si(OR) ends are sometimes referred to as alkoxy elastomers.
As regards two-component compositions, they are sold and stored in the form of two components, a first component containing the base polymer materials and the second component containing the catalyst. The two components are mixed at the time of use and the mixture crosslinks in the form of a relatively hard elastomer. These two-component compositions are well known and are in particular described in the book by Walter Noll “Chemistry and Technology of Silicones” 1968, 2nd edition, on pages 395 to 398.
These compositions essentially comprise 4 different ingredients:                a reactive α,ω-dihydroxydiorganopolysiloxane polymer,        a crosslinking agent, generally a silane, a silicate or a polysilicate,        a tin catalyst, and        water.        
Most commonly, the condensation catalyst is based on an organic tin compound. Specifically, many tin-based catalysts have already been proposed as crosslinking catalysts for these RTV-1 or RTV-2 compositions. Conventional polycondensation catalysts comprise dialkyltin compounds, in particular dialkyltin dicarboxylates such as dibutyltin dilaurate and dibutyltin diacetate, alkyl titanate compounds, such as tetrabutyl titanate or tetraisopropyl titanate, or titanium chelates (EP-A-0 885 933, U.S. Pat. No. 5,519,104, U.S. Pat. No. 4,515,932, U.S. Pat. No. 4,563,498, U.S. Pat. No. 4,528,353).
However, the alkyltin-based catalysts, although they are very effective, most commonly colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
According to another approach, catalysts of the silicone polycondensation reaction, which have a guanidine structure, such as tetramethylguanidine, described in international patent application WO 2004/020525, have been described. Other catalysts with a silylated guanidine structure have also been developed and are described, for example, in U.S. Pat. No. 4,248,993. Although these applications do not describe antifouling paints, it should be noted that the problem associated with the use of these organic catalysts is that they must be used in the presence of specific crosslinking agents that are very reactive and expensive (silanes comprising 1-methylvinyloxy functions), i.e. conventional crosslinking agents of simple structures, which are very widely used in single-component or two-component RTV formulations, for instance alkyl trialkoxysilanes, alkyl silicates or alkyl polysilicates, cannot be combined with them without the obligatory presence of a very reactive crosslinking agent such as a silane comprising 1-methylvinyloxy functions. This is because, without the presence of this very reactive silane, the crosslinking of the composition to give an elastomer is insufficient and does not make it possible to obtain good mechanical properties. Thus, when the derivative 1,1,3,3-trimethylguanidine is used with a conventional crosslinking agent, for instance an alkyl polysilicate, and without the presence of a specific reactive silane comprising a methylvinyloxy function, the crosslinking of the system is then insufficient and cannot generate a silicone elastomer.
In antifouling uses involving large amounts of paints, this problem is totally unacceptable owing to the increased cost caused by the use of a very reactive, expensive specific crosslinking agent which offers the final user little flexibility.
For sustainable development, it therefore appears to be necessary to develop novel antifouling paints which do not comprise any toxic catalysts. In addition, these catalysts should be usable irrespective of the type of crosslinking agent used and thus allow the use of crosslinking agents which are more health-and-safety friendly.
For example, an important characteristic of a curable silicone composition is the crosslinking kinetics. The time needed to obtain a dry surface (or Tack Free Time) must be short. Tack Free Times of less than one hour are generally required.
Another important characteristic of a curable silicone composition is the working time (pot-life), i.e. the time during which the composition can be used after mixing without curing. This time must be sufficiently long to allow its use, but sufficiently short to obtain a hard coating. For example, for a coating of tie coat or topcoat type, a pot-life of more than 1 hour is generally required when the external temperature is between 20 and 30° C. Now, one of the means for adjusting this pot-life is the nature of the components used, such as the catalyst.
For all these reasons, novel strategies for combating the adhesion of aquatic fouling, and in particular marine fouling, are being developed today.